Steam-driven thermal modification of coal pore structure: Insights from nuclear magnetic resonance and low-temperature nitrogen gas adsorption

This study investigates the pore structure modification of coal under thermal steam circulation injection using nuclear magnetic resonance (NMR) and low-temperature nitrogen gas adsorption techniques. Results demonstrate that steam stimulation promotes pore network development by increasing T2 spectral area (indicative of pore connectivity) and fractal dimension (reflecting surface roughness), while simultaneously inducing localized thermal stress-driven pore collapse, evidenced by permeability reduction and pore volume redistribution. Analysis of injection parameters reveals a critical trade-off: extended steam exposure enhances macropore dilation (specific surface area increased by ∼30%), yet repetitive cycles accelerate thermal efficiency decay due to steam condensation hysteresis. NMR-derived T2 cutoff values and mass balance data further quantify dynamic interactions—longer injection durations amplify mesopore connectivity (pore throat growth rate +18% vs baseline), but cyclic operations reduce effective wetting range by 15%–22% per additional cycle. Crucially, optimal steam conformance requires balancing injection duration and frequency to mitigate microscale structural damage while sustaining macro-permeability gains. These findings highlight thermal steam's dual role in coal seam gas extraction: enhancing methane desorption capacity through controlled porosity engineering, yet necessitating real-time parameter adjustments to counterbalance thermal degradation effects. The thermal modification technology using hot steam not only optimizes resource use efficiency to the fullest extent but also seamlessly aligns with China's strategic goal of carbon neutrality.